Polymer-clay nanocomposites are known. They are materials comprising a polymer matrix and exfoliated clay that is dispersed in the polymer matrix. Nanocomposites have improved mechanical properties and heat resistance compared with the base polymer. Moreover, due to the layered structures, nanocomposites have increased barrier to oxygen. This characteristic makes nanocomposites desirable candidates for the packaging materials. Thus, there is an increasing interest in developing polyolefin-clay nanocomposites since polyolefins are the most widely used packaging materials.
Clays are hydrophilic. Thus, to prepare polymer-clay composites, clays are treated with organic compounds to convert then into organoclays. Organoclays have improved compatibility with polymers, in particular, with polar polymers such as polyamide. When an organoclay is dispersed in a polymer matrix, the polymer chains insert between the adjacent layers and thereby the clay is delaminated. This process is called exfoliation.
Polyolefin-clay nanocomposites are difficult to make because organoclay and polyolefin are incompatible. To prepare polyolefin-clay nanocomposites, polymeric compatiblizing agents are required. See, e.g., www.Nanoclay.com. See also Peter Reichert et al., Macromol. Mater. Eng., 275, pp. 8-17 (2000). Maleated polyolefins are commonly used compatiblizing agents. They are expensive. Further, the addition of maleated polymers often alters the desired properties of the nanocomposites.
Shear modification of polyolefins is also known. For instance, U.S. Pat. No. 6,171,993 teaches low-shear modification of LLDPE. The modified LLDPE shows decreased melt elasticity. However, shear modification of nanocomposite is unknown. Such modification is of significant commercial value because the modification procedure is convenient and inexpensive. Ideally, the modified nanocomposites would have significantly enhanced rheological, mechanical, and barrier properties.